Anthony Rapp. Photo by Suellen Fitzsimmons.

If you're familiar with either Rapp's 2006 tell-all of the same title, or of the musical Rent with which he is so closely associated, little of what transpires during the show's 90 wrenching minutes will much surprise you. From fall of 1994 to spring of 1996, Rapp transitions from a (barely) working actor to a major star because of Jonathan Larson's hit pop-opera retelling of La Bohème. But that success does not come without its share of pain, as Larson never lives to see his creation become a phenomenon, and Rapp loses his mother to body-devouring cancer soon after the show moves to Broadway.

Were it not for Rapp's unique role in Rent's history, this show—which has been directed by Steven Maler and is backed by a hot six-piece band led by Daniel A. Weiss (and featuring killer composer Tom Kitt on the cello)—would not be news- or NYMF-worthy. But his creation of documentarian Mark Cohen, who captures on film a remarkable year in the life of his friends in Alphabet City, bestows a special gravitas on the proceedings. As Mark observed his fellow bohemians' relationships from the outside, so too does Rapp view, from a carefully dictated remove, the disintegration of the two people who played the most vital roles in his personal and professional life.

When speaking directly about either his mother or Larson, Rapp's pointed voice softens into a shrugging reverence, an acknowledgment that he owes them more than he can ever express, let alone repay. And when he adopts their vocal tics and physical mannerisms, they're clear modifications of his own personality that sear in your own spirit the impact they had on him. (His impersonations of everyone else, particularly key Rent personages like director Michael Grief, musical director Tim Weil, and costars Adam Pascal and Daphne Rubin-Vega, are more traditionally accurate.) Larson's inwardly hunched shoulders and apologetic enunciations, and mom's struggle against her body to find words after a lifetime of soft-spokenness, resonate with a bewitching power that yank you into the peculiar universe of Rapp's loss.

So adroit is Rapp at carving and animating in writing and performance these two personalities, as well as articulating the creation of one of modern Broadway's rare genuine sensations, that it's a shame to have to point out the rest of the evening does not live up to these elevated standards. But if Rapp has found two marvelous characters to anchor his musical journey, he hasn't surrounded them with a narrative or score that remotely match them in quality.

The first problem is that Without You never reaches its single concrete goal: to show how Rapp's mother and Larson were two halves of the same force that has boosted the actor to his now-vaunted status. Rapp tries to connect the dots, but stumbles in the differences (chiefly that Larson died suddenly and mom faded away over two years) and the fact that their two fates did not precisely overlap in terms of chronology. This gives the evening a chunky, episodic feel that mars the smoothness Rapp works so hard to impart.

The more daunting challenge the show faces is also its chief asset: its music. Featuring 11 selections from Rent that are heard in all or in part, the show is never able to define itself as entity outside of Larson's masterwork. It doesn't help that the additional songs—with Rapp's own lyrics, and music he wrote with three other composers—display considerably less craft and care than Larson's, equating cancer with an Old West figure ("Wild Bill") or chronicling the last few times Rapp saw his mother ("Visits to You") with no tangible musical or artistic flair, or that they tend to stop the action dead as the best theatre songs don't. Because Rent's songs represent a much higher level of theatrical commitment and achievement than we've seen from those in many subsequent (and less-successful) rock musicals, they stick out rather than blend in.

But it's genuinely damaging the way some are strewn in willy-nilly, as if it's more important shoehorn in the hits than to have them make dramatic sense. "Without You," for example, is so highly unsettling as structured here, at the centerpiece of a funeral scene, that even if it actually happened it plays as false. And the "Seasons of Love" finale, practically a plot-less encore, apparently exists only because any collection of Rent songs without that one is unthinkable.

What Without You needs is less Rent and more investment. Tantalizing hints of conflict about Rapp's family life (his parents were divorced almost his whole life) and his struggles with his sexuality are dropped in and then abandoned almost as quickly, some characters are introduced and then abandoned without serving much of a function, and there's a notable lack of a strong arc in terms of what Rapp's travails mean in the grander sense. And if you were hoping to learn some salacious or seldom-heard secrets about Rent's creation, you won't find anything very juicy here.

It's when Rapp is withhis mom and with Larson that the show says and feels the most, and that tears will stream most readily from your eyes. If Rapp can bring to the rest of Without You the emotional nakedness he does to his scenes with them, there's little doubt that this show will sing their praises as loudly as any contemporary solo show has. But right now it's struggling to find its voice whenever they aren't joining Rapp onstage.

­­Without You
Tickets online and current performance schedule at The New York Musical Theatre Festival